How to create amazing looking low poly stone wall?
There's a quick way to create this kind of low-poly stones: The Bisect tool:


On the tool panel, keep pressing on the Knife tool button to have access to the Bisect.
Select your object, go in Edit mode, select the edges you want to cut (probably select all), use the bisect.
Change the options in the Operator box (Inner or Outer to choose what part you want to cut off, Fill to keep the object closed).
Select all to redo. There might be some shortcuts to redo easily (and not loose the last Operator settings) but I don't know them.
Add some details with the knife if necessary (last picture).
Link-duplicate (AltD) the stone to only use several ones in your scene.